The present invention generally relates to a semiconductor integrated circuit and more particularly relates to a current driver circuit for use in a high-speed interface.
A differential interface using a twisted pair cable, consisting of two signal lines that are coupled to a terminal bias voltage via respective terminal resistors, is preferred to realize high-speed data transmission. In transmitting data through this interface, first, a current driver on the transmitting end supplies a current with constant amplitude into the twisted pair cable. Then, a receiver circuit on the receiving end senses a potential difference that has been generated between both terminals of the terminal resistors to have small and constant amplitude.
A current driver circuit, compatible with the IEEE 1394 communication standard defined for high-speed serial interfaces, is disclosed in U.S. Pat. No. 5,592,510 (issued to Van Brunt et al. on Jan. 7, 1997). The current driver circuit of Van Brunt et al. monitors a current output to a twisted pair cable and compensates for the output current based on the result of this monitoring so that the current has constant amplitude.
To further reduce the design rules for semiconductor processes and further cut down on the power consumed by resultant products, a supply voltage must be lowered. However, if the supply voltage Vdd is decreased with a terminal bias voltage Tpbias kept constant, then Vdd will be more proximate to Tpbias. Then, the drain-source voltage of PMOS current source transistors in a current driver will decrease and the PMOS current source transistors will reach a non-saturated region. That is to say, the transistors cannot operate anymore to supply a constant current. For that reason, an allowable range of Vdd is adversely limited to make the output current fall within a prescribed range. If Vdd is raised, on the other hand, the PMOS current source transistors will operate in a saturated region. However, the resistance of the drain region will rise, thus gradually increasing the amount of the output current flowing. To avoid such increase in the output current, there will almost no choice but providing a dedicated power supply for the PMOS current source transistors. Nevertheless, this is not a practical measure to be taken, because the dedicated power supply will increase the overall power consumption and production cost.
On the other hand, when Tpbias is determined on the receiving end, the current driver on the transmitting end regards the Tpbias value as being variable with a ground level at the receiving end, for example. In this case, if Tpbias deceases, then NMOS current source transistors in the current driver will reach the non-saturated region and can no longer operate to supply a constant current. Conversely, if Tpbias increases, then the PMOS current source transistors will reach the non-saturated region and cannot operate anymore to supply a constant current, either. Taking these points into account, the Tpbias value should fall within such a range as enabling both of these two types of current source transistors to supply the constant current. However, if Vdd is reduced, then the range shrinks.